70.His studies of ice-polished rocks in his Alpine homeland, far outside the range of present-day glaciers, led Louis Agassiz in 1873 to propose the concept of an age in which great ice sheets had existed in now currently temperate areas. 为什么不选D。when great ice sheets had existed in current temperate areas. 这个ice sheets不是在LA这个人提出之前就存在过吗?为什么不用过去完成时?而且有明显过去的提示 led ,,, in 1873之类的, 求指点,谢谢!
看了一个XDF老师的解释,说一般现在时不与过去完成时混用,我的理解是此处强调的时间顺序是从句中的an age in which (great ice sheets existed in what are now temperate areas),现在与过去,而不是LA这个人提出这个概念和great ice sheets存在的时间的先后顺序~